ISOMETRIC FORCE AND MAXIMAL SHORTENING VELOCITY OF SINGLE MUSCLE-FIBERS FROM ELITE MASTER RUNNERS

Single chemically permeabilized gastrocnemius fibers from six elite en durance-trained master runners (RUN group) and five age-matched sedent ary controls (SED group) were mounted between a force transducer and a position motor, studied under conditions of maximal and submaximal Ca 2+ activation, and subsequently electrophoresed on 5% polyacrylamide g els to determine myosin heavy chain (MHC) composition. For the SED gro up, peak isometric tension (P-o) averaged 143 +/- 3, 156 +/- 4, and 17 0 +/- 4 kN/m(2) and maximal shortening velocity (V-o) averaged 0.43 +/ - 0.01, 1.90 +/- 0.08 and 5.59 +/- 0.40 fiber lengths/s for fibers exp ressing type I, IIa, and IIx MHC, respectively (all comparisons, P < 0 .05). Hill plot analysis of relative forces during submaximal Ca2+ act ivation indicated no SED vs. differences in Ca2+ sensitivity or in the cooperativity of Ca2+ activation. However, at maximal Ca2+ activation , RUN type I and IIa fibers produced 15% less peak absolute force than SED fibers (P < 0.05). This reduction in fiber force was a direct res ult of the smaller diameter of the RUN fibers (P < 0.05), because P-o, peak elastic modulus (E(o)), and P-o/E(o) were not different between SED and RUN groups. RUN type I fibers also displayed a mean V-o that w as 19% higher than the average V-o of the SED type I fibers (P < 0.05) . In separate experiments, quantification of relative myosin light cha in (MLC) isoform content revealed a 28% greater ratio of MLC(3) to MLC (2) in single type I fibers from the RUN group (P < 0.05), suggesting that the elevated V-o of the RUN type I fibers was related to a greate r expression of MLC(3). In conclusion, the single fibers from the elit e master runners displayed specific morphological and contractile prop erties that may enhance the performance of these athletes during prolo nged muscular activity.